Science Centres: Aquatic Biodiversity and Biosecurity

A patch of sponges in Kawau Bay.

NIWA’s National Centre for Aquatic Biodiversity & Biosecurity is investigating how we can speed up mapping the distribution of life on the seafloor to better meet the needs of those managing our marine environment.

The distribution of habitats across the seafloor plays an important role in the functioning and diversity of marine assemblages. Resource managers, conservation biologists, and biodiversity scientists are all concerned about how fragmented or connected habitats are. For example, assessments of habitat structure can help to determine any large-scale effects on biodiversity.

Traditional quantitative sampling, which uses grabs, cores, and even video, is not cost-effective over large subtidal areas of the seafloor. However, there are now devices capable of quickly sampling such areas. These devices, which often use sound, were initially used to map sedimentary features, but they are now increasingly being used to map biological habitats on the seafloor.

Acoustic devices send out a signal and measure its energy when it is reflected back, but the slope, roughness, and absorption characteristics of the seafloor all affect this value. Seaweeds and bottom-dwelling animals will also have an effect, but how much? And how do we interpret any differences in these measurements?

There are two possible ways to map seafloor assemblages by using remote acoustic devices.

• Habitats are mapped from the acoustic data and verified by biological sampling. Although this may work for some large animals and seaweeds, the acoustical habitats may not reflect the finer-scale distributions of bottom-dwelling animals.
• Another approach is to find out what parts of the acoustic data can help to predict seafloor communities, and create a map by combining those data with values from the restricted grab, core, or video samples.

To investigate these different approaches we used a towed video camera, a single-beam sonar with the QTC VIEW™ data-acquisition system, and side-scan sonar to collect data at five sites in Kawau Bay, a large embayment on the northeast coast of the North Island. The embayment consists mainly of diverse soft-sediment habitats between 10 and 20 m deep. There are dense but patchy areas of horse mussels, sponges, tubeworms, coralline algae, and sea snails, as well as some hydroids, sea-stars, bryozoans, ascidians, and crabs. Our video data included counts of the plants and animals on the seafloor and an assessment of sediment characteristics. We found five distinct communities based on analysis of these counts.

Forty-five percent of the samples were classified into the correct single-beam defined habitats by using mud and coarse sand content. However, we were still 70% wrong in our classification of samples from one of the habitats, despite including sediment and biological information from the video. We used horse mussel, mud, and coralline algae content to classify 63% of the samples into the correct side-scan habitats.

Not all of the habitats defined by side-scan or single-beam methods had distinct assemblages, and those in each habitat were not very consistent. Descriptions of the assemblages were based on a few faunal types and did not vary much between habitats. Even densities of large species (e.g., horse mussels) did not vary greatly between acoustically defined habitats. So, although the side-scan and single-beam habitats were related to features seen on the video, they did not do a good job of describing the ecological communities.

What more needs to be done before these mapping techniques can be used on a routine basis?

• We need to study what aspects of acoustic and environmental data are useful to predict levels of biodiversity in different locations.
• Extend the work to infaunal (burrowing) assemblages.
• Determine the best way to produce broad-scale maps from highly detailed data.
• Use different techniques to develop guidelines for the amount of area that needs to be sampled.
• Update designs as new techniques become available.

This work was funded by the Foundation for Research, Science & Technology.

Judi Hewitt [ j.hewitt@niwa.co.nz ]